I can get the machine mentioned in subject, but at first I wanna know some details before I try to wipe out HP-UX and installing it with Gentoo. The architecture is hppa, right? Inside this 9000 is a FC60 SureStore DiskArray Controller which server 3 shelfs with SCSI discs: I don't think that the driver for it is open source, but is there another way to get this FC60 working?

The architecture of that machine is hppa, that's right, it's a 64 bit machine with 1-4 CPUs ranging from 360 MHz to 1 GHz.
If it's a L2000-36 or L2000-44, then it will run (tested by pateam, see http://www.pateam.org/list.html), the -5X -7x -8x variants should work as well, the -9x variant will be somewhat unstable in SMP use due to caching issues.
I cannot say anything about the FC60 controller, but the normal Symbios Logic 53C875 and Symbios Logic 53C896 SCSI controllers should not be a problem.